III-nitride multi-channel heterojunction interdigitated rectifier

ABSTRACT

A III-nitride power semiconductor device that includes a plurality of III-nitride heterojunctions.

RELATED APPLICATION

This application is based on and claims benefit of U.S. ProvisionalApplication No. 60/614,675, filed on Sep. 30, 2004, entitled III-NitrideMulti-Channel Heterojunction Interdigitated Rectifier, to which a claimof priority is hereby made and the disclosure of which is incorporatedby reference.

BACKGROUND OF THE INVENTION

The present invention relates to power semiconductor devices and moreparticularly to heterojunction power semiconductor devices.

III-nitride heterojunction power semiconductor devices are desirable forpower applications due to their high breakdown capability, and low ONresistance. U.S. patent application Ser. No. 11/004,212, assigned to theassignee of the present application illustrates an example of aIII-nitride power semiconductor device.

Referring to FIG. 2, a III-nitride power semiconductor device as shownin U.S. patent application Ser. No. 11/004,212 includes substrate 28,buffer layer 30 disposed on substrate 28, a heterojunction 32 disposedon buffer layer 30, a protective layer 34 disposed on heterojunction 32,a schottky electrode 20 in schottky contact with heterojunction 32, andan ohmic contact 22 ohmically connected to heterojunction 32.Preferably, schottky contact 20 and ohmic contact 22 both include afield plate 36.

Heterojunction 32 includes a resistive III-nitride semiconductor body(resistive body) 38 and a III-nitride semiconductor barrier body(barrier body) 40 both formed with an alloy of InAlGaN. Resistive body38 and barrier body 40 are selected so that the junction between the twocreates a highly conductive two dimensional gas (2DEG) 42 due tospontaneous polarization and the piezoelectric effect as is well knownin the art.

One known material for forming resistive body 38 is undoped GaN, and aknown material for forming barrier body 40 is AlGaN.

SUMMARY OF THE INVENTION

A power semiconductor device according to the present invention includesat least a first III-nitride heterojunction and at least a secondIII-nitride heterojunction disposed over the first III-nitrideheterojunction. As a result, a device according to the present inventionincludes a number of high density, high mobility 2DEG channels.

Specifically, each heterojunction is preferably formed with a thin firstIII-nitride semiconductor body of one InAlGaN alloy and a second thinsemiconductor body of another InAlGaN alloy. For example, each layer canbe between 10-1000 Å, and preferably between 150-300 Å. The multi layersof thin, but highly conductive heterojunction bodies result in a highlyconductive power semiconductor device with a relatively high breakdownvoltage.

A device according to the preferred embodiment is a lateral channelschottky type rectifier which includes schottky electrodes and ohmicelectrodes alternately arranged in an interdigitated pattern resemblinga comb to increase charge injection and extraction. The electrodes inthe preferred embodiment are preferably disposed on the stack of atleast two III-nitride heterojunctions. An example of such a structure isa stack of AlGaN/GaN/AlGaN/GaN disposed over a buffer layer and asubstrate. Preferably, at least the bottom GaN layer is highlyresistive; i.e. intrinsically doped, which means that it contains nomore than residual doping, and is considered unintentionally doped.

In the example described above, under forward bias, the channel formedat the AlGaN/GaN interface can carry very large currents without the useof a thick doped region. In reverse bias, the channel is depleted ofmobile charge, so that no current can flow in the channel, and thehighly resistive nature of the underlying GaN prevents charge fromflowing. In addition, when the AlGaN and the GaN are intrinsically dopedlow electric fields result under a reverse bias, allowing for very highstandoff voltages without the corresponding adverse effects on theforward resistance. It should be noted that, depending on the intendeddevice characteristics, the layers can be doped to obtain the desiredtrade off between mobility and breakdown ability.

Also, advantageously, the replacement of a doped current carrying layerwith a highly conductive 2DEG drastically improves the RA product for agiven breakdown voltage. Furthermore, the use of multiple layers ofAlGaN/GaN results in increased current conduction, and the resistive GaNlayer allows isolation of the device by etching away, for example,portions of AlGaN-GaN stack that surround the device. As a result it maybe possible to integrate a number of devices on a single chip, therebyallowing for the fabrication of IC's that include a device according tothe present invention.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically illustrates a cross-sectional view of a powerdevice according to the prior art.

FIG. 2 shows a top plan view of a portion of an active cell of a deviceaccording to the present invention.

FIG. 3 shows a cross-sectional view of a portion of an active cell of adevice according to an embodiment of the present invention along lineA-A in FIG. 2 as seen in the direction of the arrows.

FIGS. 4A-4D schematically illustrate the fabrication of a deviceaccording to the present invention.

FIG. 5 schematically shows a cross-sectional view of a portion of adevice according to an alternative embodiment of the present invention.

FIG. 6 schematically shows a variation of the embodiment shown in FIG.5.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring to FIG. 2, a device according to an embodiment of the presentinvention includes a plurality of interdigitated power electrodes,namely schottky electrodes 20 and ohmic electrodes 22. Schottkyelectrodes 20 are connected to a common schottky feed 24 and ohmicelectrodes 22 are connected to a common ohmic feed 26. Although notshown, one skilled in the art would understand that each common feed 24,26 is connected electrically to a respective conductive pad for externalconnection.

A device according to the present invention includes two or moreIII-nitride heterojunctions. Referring, for example, to FIG. 3, a powerdevice according to the present invention includes anotherheterojunction 33 between heterojunction 32 and buffer layer 30.Heterojunction 33 preferably includes a III-nitride semiconductorresistive body (resistive body) 39, and a III-nitride semiconductorbarrier body (barrier body) 41. Resistive body 39 and barrier body 41are selected so that the junction between the two creates a highlyconductive 2-DEG 42 as described above. In the preferred embodiment,resistive body 39 may be composed of undoped GaN, while barrier body 41is preferably composed of AlGaN. It should be noted that other alloys ofInAlGaN can be used to form either resistive body 39 or barrier body 41without deviating from the spirit and the scope of the presentinvention.

In the preferred embodiment, substrate 28 is formed from silicon. Itshould be noted that substrate 28 can also be formed with SiC, sapphire,or any other suitable substrate. In addition, a compatible III-nitridesemiconductor bulk material such as bulk GaN can be used as substrate28, in which case, buffer layer 26 may be omitted when resistive body 38is composed of intrinsic GaN.

Referring to FIG. 4A, in order to fabricate a device according to thepresent invention a stack including substrate 28, buffer layer 30,heterojunction 32, heterojunction 33, and protective layer 34 is maskedand etched to provide an opening 44 in protective layer 34 asillustrated schematically in FIG. 4B. Opening 44 reaches at leastbarrier body 40 of heterojunction 32.

Next, ohmic electrode 22 is formed in opening 44 reaching at least andmaking ohmic contact with barrier body 40, resulting in the structureschematically illustrated by FIG. 4C. Ohmic electrode 22 is formed bydepositing an ohmic metal followed by an annealing step. Thereafter,another opening 46 is formed in protective layer 34, opening 46 reachingat least barrier body 40, as shown schematically in FIG. 4D. Next,schottky electrode 20 is formed by depositing a schottky metal inopening 46 reaching at least and making schottky contact with barrierbody 40. Field plate 36 is then formed atop schottky electrode 20,resulting in a device according to the embodiment shown in FIG. 3.

Referring next to FIG. 5, a device according to another embodimentincludes a lightly doped III-nitride body 48, formed preferably fromGaN, and disposed over heterojunctions 32, 33. In this embodiment, ohmicelectrode 22 and schottky electrode 20 are connected to lightly dopedIII-nitride body 48. This structure is similar to a lateral conductionschottky, but exhibits an increased conductivity in the forward currentcarry direction.

Referring to FIG. 6, in a modified version of the embodiment shown inFIG. 5, a recess is provided in lightly doped III-nitride body 48 toallow ohmic electrode 22 to be positioned on a plane below the plane ofschottky electrode 20 and to ohmically connect to heterojunction 32.

It should be noted that protective layer 34 can be made from a materialthat retards or prevents the out diffusion of nitrogen during theannealing step. A nitrogen rich material may be suitable for thispurpose. For example, AlN, HfN, AlGaN, highly doped GaN, highly dopedpoly GaN, and LPCVD Si₃N₄ are among materials suitable for formingprotective layer 34. Also, in the preferred embodiment ohmic electrode22 may be formed from an Al/Ti body such as Ti/Al/Ti/TiW, Ti/Al/Ni/Au,or the like, while schottky electrode 20 may be formed from Ni/Au,Pt/Au, Pd/Au, Au, TiW, Ni/TiW, Ni/TiN, or the like. It should be notedthat the materials noted herein are preferred, but that other materialsmay be used without deviating from the scope and spirit of the presentinvention.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

1. A power semiconductor device comprising: a first III-nitrideheterojunction; a second III-nitride heterojunction arranged on saidfirst III-nitride heterojunction; a first power electrode electricallyconnected to said second III-nitride heterojunction; and a second powerelectrode spaced from said first power electrode and electricallyconnected to said second III-nitride heterojunction.
 2. The device ofclaim 1, wherein said first power electrode makes schottky contact withsaid second III-nitride heterojunction, and said second power electrodemakes ohmic contact with said second III-nitride heterojunction.
 3. Thedevice of claim 1, wherein each heterojunction includes a firstIII-nitride semiconductor body comprised of an alloy of InAlGaN, and asecond III-nitride semiconductor body comprised of another alloy ofInAlGaN.
 4. The device of claim 3, wherein said first semiconductor bodyis comprised of AlGaN, and said second semiconductor body is comprisedof GaN.
 5. The device of claim 3, wherein said second semiconductor bodyis undoped.
 6. The device of claim 1, further comprising a buffer layerand a substrate, said buffer layer being disposed between said firstIII-nitride heterojunction and said substrate.
 7. The device of claim 6,wherein said buffer layer is comprised of AlN.
 8. The device of claim 6,wherein said substrate is Si, SiC, or Sapphire.
 9. The device of claim1, further including a substrate comprised of a III-nitride compatiblematerial.
 10. The device of claim 9, wherein said substrate is comprisedof GaN.
 11. The device of claim 1, wherein said first III-nitrideheterojunction includes a first III-nitride semiconductor body comprisedof one alloy of InAlGaN, and a second undoped III-nitride semiconductorbody comprised of another alloy of InAlGaN and disposed over a bufferlayer.
 12. The device of claim 11, wherein said buffer layer iscomprised of AlN.
 13. The device of claim 11, wherein said secondundoped III-nitride semiconductor body is comprised of GaN.
 14. Thedevice of claim 13, wherein said first III-nitride semiconductor body iscomprised of AlGaN.
 15. The device of claim 1, further comprising aprotective layer disposed over said second III-nitride heterojunction.16. The device of claim 1, wherein said first power electrode isconnected to a first electrical feed, and said second power electrode isconnected to a second electrical feed.
 17. The device of claim 1,wherein said first power electrode and said second power electrode areinterdigitated.
 18. The device of claim 15, wherein said protectivelayer prevents the out diffusion of nitrogen.
 19. The device of claim15, wherein said protective layer is nitrogen rich.
 20. The device ofclaim 15, wherein said protective layer is comprised of at least one ofAlN, HfN, AlGaN, highly doped GaN, highly doped poly GaN, and LPCVDSi₃N₄.
 21. The device of claim 1, further comprising a lightly dopedIII-nitride body interposed at least between said first power electrodeand said second III-nitride heterojunction.
 22. The device of claim 1,further comprising a lightly doped III-nitride body interposed betweensaid first and said second power electrodes.
 23. The device of claim 21,wherein said lightly doped III-nitride body is comprised of GaN.
 24. Thedevice of claim 22, wherein said lightly doped III-nitride body iscomprised of GaN.